I've been using one for about a year. It is excellent, but the battery life is limited - about 5 hours at best. You can leave it connected to a USB power supply (plugged into a cigarette lighter adapter) and it will last indefinitely, though in my experience it needs to be restarted about once every 24 hours, or it seems to stop working (memory leak?).
I have been amazed by how good the coverage is - I was staying on the beach in NE Scotland last month - where the 3 map says there is no coverage, and was able to get it to work by putting it up in the skylight.
There have been places where there was no signal at all, but they were rare!